Solenoid-operated devices, and especially dc-powered solenoid-operated devices, are widely used in power plants, nuclear facility plant safety systems, control rod latch mechanisms of research reactors, and many other industrial applications.
Monitoring the position and/or movement of a solenoid armature is fundamental in detecting the degradation of control mechanisms used within nuclear power plants. For example, as solenoid-operated valves age the actuated plungers experience changes in the movement associated with their functions. In fact, plungers sometimes "freeze" within the solenoid assembly, and do not move when the solenoid is properly energized and deenergized. "Freezing" of solenoid plungers may occur for a variety of reasons including changes in the properties of elastomeric seats, worn O-rings, contaminants, or faulty valve assemblies. Solenoid-operated valves are commonly found in devices such as containment isolation valve actuators, boiling-water reactor control-rod scram systems and pressurized-water-reactor safety injection systems. (For a more complete listing see R. C. Kryter, Aging and Service Wear of Solenoid-Operated Valves Used in Safety Systems of Nuclear Power Plants Evaluation of Monitoring Methods, NUREG/CR-4819, ORNL/TM-12038, Vol. 2, July 1992, p.2.)
A need exists for a method and apparatus capable of detecting degradation in control latch mechanisms of a new research reactor. A need also existed for a system to detect degradation of solenoid-operated control rod drives in forty-seven operating power plants that use this type of drive system. In addition to the concerns associated with the degradation of valves in nuclear power plants, many other industrial sectors utilize solenoid-operated valves and express similar concerns.
Some of these problems have been addressed in the prior art. However, these prior techniques have not been completely successful in confronting the industrial problems associated with monitoring armature position in solenoid-operated devices. For example, in solenoid-operated devices driven by an alternating current (ac), the motion of the solenoid armature, as the solenoid is energized, can be detected by monitoring the change in impedance to the flow of the alternating current through the coil of the solenoid. Characterization of the change in impedance as a function of time during the armature's stroke may be used to distinguish normal operation of the device from operation where binding or wear of the moving parts has taken place.
Further, direct current (dc) powered solenoids are widely used in nuclear power plants for control rod actuation and for solenoid-operated valves. Detecting degradation in both of these driven devices is a requisite to safe operation.
As stated above, the necessity of monitoring armature position and/or movement in solenoid-operated valves is fundamental to proper system maintenance. This is especially so in nuclear power plants. For example, an evaluation of monitoring methods to detect aging and service wear of solenoid-operated valves (SOV's) used in the safety systems of nuclear power plants was carried out by R. C. Kryter of ORNL's Instrumentation and Controls Division for the U.S. Nuclear Regulatory Commission (NRC). The experimental work demonstrated the ability to detect ac-powered solenoid armature movement and position based upon impedance measurements. However, the report states that " . . . the method is applicable only to ac-powered SOVs" and "dc-powered valves show no corresponding change in their terminal resistance as the plunger changes its position within the solenoid coil, . . . " R. C. Kryter, supra.
The rod control drive system in Westinghouse pressurized water reactors utilizes three dc-powered solenoids in each drive to provide motive and clamping actions. An aging assessment of these drives was carried out by Brookhaven National Laboratory (BNL) for the NRC. In that assessment, various inspection, surveillance, monitoring and maintenance techniques are discussed, including a system developed by the Japanese that uses on-line analysis of current signals and acoustic noise generated by rod movement. W. Grunther and K. Sullivan Aging Assessment of the Westinghouse PWR Control Rod Drive System, NUREG/CR, BNL NUREG 52232, March 1990 (Draft), p.6-14. That system, used to indicate preventive maintenance needs, develops a trace of coil current versus time during coil energization. Movement of the solenoid armature induces a perturbation in the current trace at the time movement occurs due to generation of back emf that partially counters the supplied current. However, anomalous behavior along the length of the stroke is not detectable by this system. The BNL report also mentions the use of a current signature analysis technique at one plant, in which the current to each coil is traced during rod motion Id at p. 8--8. It is asserted that this technique determines the acceptability of the power circuitry, the logic circuitry and the coil integrity, as well as providing a rough indication of mechanical interferences. Further details are not provided. Id.
As demonstrated in U.S. Pat. No. 4,950,985, to Voss et al., attempts have been made in the prior art to monitor the position of an armature in a coil/armature magnetic system. Specifically, the patent discloses an electromagnet having a dc-powered magnet with a coil, an armature or plunger which reciprocates within and along the axis of the coil, and an impulse generator which introduces an exciting frequency into the resonant circuit of the system. The resonant frequency of the apparatus is "identified by an evaluator which includes any device capable of recognizing and interpreting the various signals from the resonant circuit in response to the test signal introduced into the resonant circuit and to determine on the basis of these response signal the position of the armature." (Col 2, lines 60-66).
The Voss patent, however, fails to disclose the introduction of an alternating current flow into the solenoid coil to monitor armature position during both energization and deenergization of the solenoid-operated apparatus. As a result, the Voss patent is not capable of monitoring armature position based upon the impedance and resistance of the solenoid coil.
The use of ac and dc energy for the purpose of providing a proximity sensor that has reduced temperature sensitivity is proposed in U.S. Pat. No. 5,180,978, to Postma et al.. The Postma proximity sensor relies upon the application of a high frequency alternating current through the coil winding. In Postma, as the sensed device is moved toward and away from the sensor, the ac resistance and dc resistance are measured to determine the position of the sensed apparatus. In addition, the Postma sensor also includes a system to correct for changes in temperature that might effect necessary calculations.
The patents to Bean et al. (U.S. Pat. No. 5,115,193), Fischer (U.S. Pat. No. 5,045,786), Granber et al. (U.S. Pat. No. 4,810,964), and Larson et al. (U.S. Pat. No. 4,112,365) disclose other electromagnetic devices utilizing systems for monitoring conditions that are relevant to the operation of the electromagnetic apparatuses.
In view of the complex prior art systems discussed above, there is a need for a simple and reliable system that will allow a user to monitor the position of a solenoid's armature whether the solenoid is energized or deenergized.